The related U.S. patent, Transducer Apparatus and Method for Intravascular Blood Flow Measurement”, assigned to the same assignee as the present invention, discusses the importance of measuring blood flow in a non-invasive manner. As disclosed tissue viability depends upon adequate blood flow in the feeding vessels. Doppler ultrasound is widely used for such measurement, and is well understood; see, for example, “Doppler Ultrasound”, by Evans and McDicken, as is disclosed in the aforementioned related patent application.
As described in the aforementioned related patent application, minimally-invasively placing Doppler transducers within the blood vessels, e.g. by means of catheters, is desirable. The diffraction-grating transducer described in the referenced application provides apparatus and methods to accomplish such measurement. In the referenced disclosure, diffraction grating ultrasonic transducers are used to produce beams of ultrasound that are backscattered by moving blood so that Doppler measurement methods may be used to calculate the blood velocity, and thus determine blood flow.
In order to spread their beams over the area of the blood vessel, the transducers, taught in the aforementioned related patent application, depend upon their narrow width to cause diffraction. As the amount of ultrasound scattered by blood is inversely proportional to wavelength, small wavelength (high frequency) is desirable for low power requirements. Thus, those transducers can only be a few, e.g. 2 or less, wavelengths wide. This limitation restricts the devices disclosed therein to being one or more long extended ribbon-like transducers.
When using plastic piezoelectrics, as described in the aforementioned related patent application, unless the transducers are quite long, or multiple transducers are used, the impedance levels of the transducers make matching to the sending and receiving electronics, as is well known a requirement for efficient transmitting and reception, difficult.
Moreover, these transducers must be patterned, i.e. have alternately reversed polarity sections, in order to form the diffraction-grating transducer, of the spacing corresponding, for example, to two acoustic wavelengths. For example, a repeating pattern of 150 microns is required for 20 MHz ultrasound. While not in any way straining the capability of photolithographic art, such patterning in metal requires masking and other procedures that may make fabrication of these transducers not inexpensive.